The present invention relates to an apparatus for growing a single crystal used for production of single crystals such as silicon single crystals by the Czochralski method (also referred to as the xe2x80x9cCZ methodxe2x80x9d or xe2x80x9cpulling methodxe2x80x9d hereinafter), a production method and a single crystal.
Hereafter, conventional art relating to the present invention will be explained by exemplifying growing of a silicon single crystal.
An apparatus for growing a single crystal used for producing a silicon single crystal by the CZ method generally comprises a crucible accommodating a raw material melt, which can be moved upwardly and downwardly, and a heater disposed so as to surround the crucible, both of which are provided in a main chamber for growing a single crystal, and a pulling chamber for accommodating and taking out a grown single crystal is continuously provided above the main chamber. When a single crystal is produced by using such an apparatus for growing a single crystal, a seed crystal is immersed in the raw material melt and carefully pulled upwardly with rotation to grow a rod-like single crystal, while the crucible is moved upwardly according to the growth of the crystal so that the melt surface should be always maintained at a constant height in order to obtain desired crystal quality.
Further, when the single crystal is grown, the seed crystal attached to a seed holder is immersed in the raw material melt, and then the seed crystal is pulled upwardly with rotation in a desired direction by carefully winding up a wire by means of a pulling mechanism to grow a single crystal ingot at the end of the seed crystal. In this case, in order to eliminate dislocations produced when the seed crystal is brought into contact with the melt, the crystal in an early stage of the growth is once made thin to a small diameter of about 3 to 5 mm, and then the diameter is increased after the dislocation are eliminated so as to grow a single crystal ingot of desired quality.
At this time, the pulling rate for a portion having a constant diameter of the single crystal ingot is usually extremely slow, i.e., about 0.5 to 1 mm/min, and if it is pulled fast by constraint, there arisen problems, for example, the growing single crystal is deformed and thus a cylindrical product having a constant diameter can no longer be obtained, slip dislocations are generated in the single crystal ingot, the crystal is detached from the melt and thus it cannot be a product and so forth. Therefore, increase of the crystal growing rate is limited.
However, for the purpose of improving productivity and reducing cost in the production of single crystal ingots by the aforementioned CZ method, increase of the single crystal growth rate is one of considerable means, and various improvement have hitherto been made to achieve increase of the single crystal growth rate.
The pulling rate, i.e., the single crystal growth rate is determined by the heat balance of the growing crystal. The heat quantity incorporated into the crystal consists of inflow heat quantity from the melt and the heater and solidification latent heat generated when the melt crystallizes. When the heat balance of the growing crystal is considered, it is necessary that outflow heat quantity emitted out of the crystal from the crystal surface and the seed crystal should be equal to the sum of the inflow heat quantity and the solidification latent heat. The solidification latent heat depends on the volume of the crystal growing per unit time. Therefore, in order to increase the crystal growth rate, it is necessary to compensate increase of solidification latent heat provided by increase of the crystal growth rate by reducing the inflow heat quantity or increasing the outflow heat quantity.
Therefore, it is generally used a method of efficiently removing heat emitted from the crystal surface to increase the outflow heat quantity.
As one of such means, there was proposed apparatus in which the pulling rate is increased by providing cooling means in the main chamber so as to surround a single crystal ingot under pulling and thereby efficiently cooling the single crystal ingot under pulling. For example, there is the apparatus disclosed in Japanese Patent Laid-open (Kokai) Publication No. 6-211589. In this apparatus, a gas flow guide cooling cylinder having a double structure consisting of an outer cooling cylinder composed of metal and an inner cooling cylinder composed of graphite or the like is provided from the bottom portion of the pulling chamber to the inside of main chamber so as to concentrically surround a single crystal ingot under pulling and thereby heat generated in the inner cooling cylinder is transferred to the outside by the outer cooling cylinder, so that temperature increase of the inner cooling cylinder should be suppressed and cooling efficiency of the crystal should be improved.
Apparatuses utilizing cooling medium such as water in order to more efficiently cool a growing single crystal are also disclosed. For example, in the apparatus for growing a single crystal disclosed in Japanese Patent Laid-open (Kokai) Publication No. 8-239291, a cooling duct for circulating a liquid refrigerant is provided in a main chamber and a cooling member composed of a material having high heat conductivity such as silver is provided below the duct so as to rapidly transfer heat emitted from crystal surface to the outside and thereby attain effective cooling of crystal. However, if fluid such as water generally used as the cooling medium approaches the melt surface heated to a high temperature exceeding 1000xc2x0 C., it may be a cause of phreatic explosion and thus dangerous. Therefore, in this apparatus, safety is secured by separating the cooling duct from the melt surface.
In these apparatuses, for example, in the aforementioned apparatus disclosed in Japanese Patent Laid-open (Kokai) Publication No. 6-211589, the outer cooling cylinder composed of metal and the inner cooling cylinder composed of graphite or the like in the double structure of the cooling cylinder show a difference in coefficient of thermal expansion and they are impossible to be always in perfect contact with each other. As for the disclosed apparatus, it is described that the diameter is made gradually smaller toward the downward direction so as to secure a larger contact area. However, even in such a case, they are actually contacted in a line and perfect contact cannot be obtained. Therefore, in an actual practice, a gap is formed between the outer cooling cylinder and the inner cooling cylinder and it acts as a heat insulating layer. Furthermore, there exists contact thermal resistance between the outer cooling cylinder and the inner cooling cylinder. This contact thermal resistance depends on type of material and surface condition, and it cannot be easily determined. However, in the structure used in the disclosed apparatus, the inner cooling cylinder cannot be cooled sufficiently and thus there is a problem that it is still impossible to exert significant crystal cooling effect.
On the other hand, as for the apparatus for growing a single crystal disclosed in Japanese Patent Laid-open (Kokai) Publication No. 8-239291, the cooling duct and the melt surface are separated and thus attention is paid for safety. However, in such a structure, the whole cooling duct is disposed at approximate center of the inside of the main chamber, and it causes problems concerning workability and operability in practical use. Further, it is difficult to secure sufficient strength of the duct due to its structure, and it is expected that the risk of leakage of liquid refrigerant due to breakage of the duct would increase.
Furthermore, there is an area between the cooling duct and the pulling chamber where the crystal is not sufficiently cooled. Therefore, the apparatus cannot always provide efficient removal of the outflow heat quantity emitted from the crystal, and it cannot be considered sufficient for obtaining significant cooling effect.
In view of the aforementioned problems, an object of the present invention is to provide an apparatus for growing a single crystal that can exert cooling effect on a grown single crystal to the maximum extent so as to accelerate the crystal growth rate and safely produce a single crystal without leakage of cooling medium due to breakage, melt down etc., as well as a method for producing a single crystal utilizing such an apparatus and a single crystal produced by utilizing such an apparatus.
In order to attain the aforementioned object, the apparatus for growing a single crystal of the present invention is an apparatus for growing a single crystal comprising at least a main chamber enclosing a crucible for accommodating a raw material melt and a heater for heating the raw material melt and a pulling chamber continuously provided above the main chamber, into which a grown single crystal is pulled and stored, wherein the apparatus further comprises a cooling cylinder that extends at least from a ceiling of the main chamber toward a raw material melt surface so as to surround a single crystal under pulling and is forcibly cooled with a cooling medium, and an auxiliary cooling member extending below the cooling cylinder and having a cylindrical shape or a shape tapered toward the downward direction.
If a cooling cylinder that is forcibly cooled with a cooling medium, and an auxiliary cooling member that downwardly extends from the cooling cylinder are provided as described above, the heat radiation from the heater is shielded by the auxiliary cooling member so that the single crystal at an extremely high temperature pulled from the raw material melt is gradually cooled and further effectively cooled by the cooling cylinder as it moves upwardly. As a result, it becomes possible to increase the growth rate of the crystal. In particular, the structure where the cooling cylinder that extends from the ceiling of the main chamber toward the raw material melt surface is forcibly cooed with a cooling medium improves the cooling capacity and enables cooling of crystal by effectively utilizing the space in the upper part of the main chamber. Therefore, it becomes possible to use a longer region where the crystal can be forcibly cooled and thus higher crystal cooling effect can be obtained.
Further, since the cooling cylinder is separated from the melt surface at an extremely high temperature by a sufficient distance, the melt would not be brought into contact with the cooling cylinder due to scattering of melt caused during the melting operation of the raw material or caused by rarely happening earthquake or the like and thus breakage or melt down thereof is not caused. Therefore, a single crystal can be grown very safely.
In the apparatus for growing a single crystal according to the present invention, the aforementioned cooling cylinder is preferably composed of iron, nickel, chromium, copper, titanium, molybdenum, tungsten or an alloy containing any one of these metals, or any of the aforementioned metals and alloy coated with titanium, molybdenum, tungsten or a platinum group metal.
The aforementioned metals are excellent in heat resistance and heat conductivity. Therefore, if these metals are used for the apparatus of the present invention, it can absorb the radiant heat from the heater and the melt surface and efficiently transfer it to the cooling medium such as water.
Further, the aforementioned auxiliary cooling member preferably consists of graphite, molybdenum or tungsten.
These materials are extremely excellent in heat resistance, and therefore they can effectively shield the radiant heat from the melt and the heater and are also suitable for cooling a single crystal ingot at an extremely high temperature immediately after the pulling from the melt surface. Further, if the auxiliary cooling member is constituted with such materials, it can be an auxiliary member also excellent in durability, and it is scarcely deformed or suffers from distortion at a high temperature. Therefore, it can be used for a long period of time, and because of its high mechanical strength, the handling at the time of dismounting or cleaning of the apparatus for growing a single crystal is concurrently becomes easy and workability is also improved.
Furthermore, a heat-shielding member is preferably provided to the aforementioned auxiliary cooling member.
If a heat-shielding member is provided to the auxiliary cooling member as described above, the radiant heat from the heater and the melt can be more effectively shielded, and as a result, the crystal growth rate can be further improved.
The end of the aforementioned cooling cylinder is preferably separated from the surface of the raw material melt contained in the crucible by 10 cm or more.
If the cooling cylinder is disposed with a predetermined distance from the raw material melt surface as described above, the risk of adhesion of the melt to the cooling cylinder should be substantially eliminated, and therefore a single crystal can be grown more safely. Further, by providing a distance of 10 cm or more between the melt surface and the lower end of the cooling cylinder, a fusion ring at a growing position of the crystal can be easily observed from the outside of the apparatus without any particular processing of the cooling cylinder, and it is sufficient to provide the auxiliary cooling member that can be easily processed with a detection window for control of crystal diameter and so forth. Therefore, the apparatus is simplified and it becomes possible to perform stable operation.
A protection member consisting of graphite or metal is preferably provided outside the aforementioned cooling cylinder.
By providing a protection member having heat resistance outside the cooling cylinder, adhesion of the melt scattered during the melting operation of the raw material and so forth to the cooling cylinder or deposition of material vaporized from the raw material melt during the operation on the surface of the cooling cylinder can be suppressed. This makes it possible to perform stable operation for a long period of time. Further, the radiant heat from the heater and the raw material melt surface can also be prevented from directly irradiating the cooling cylinder, and therefore it becomes possible to obtain further effective cooling effect.
Furthermore, the inner surface of the cooling cylinder is desirably subjected to a blackening treatment.
By subjecting the inner surface of the cooling cylinder to a blackening treatment such as application or vapor deposition of graphite or the like, absorption efficiency for the heat radiated from the crystal can be further improved. Increase of heat absorption efficiency of the cooling cylinder provided by the blackening treatment can realize faster crystal growth.
According to the present invention, there is further provided a method for producing a single crystal wherein a single crystal is grown by using the aforementioned apparatus for growing a single crystal, and a single crystal wherein it is grown by using the aforementioned apparatus for growing a single crystal.
By using the apparatus for growing a single crystal according to the present invention, a cylindrical silicon single crystal can be safely grown at a higher crystal growth rate without deformation of the crystal. Therefore, it also becomes possible to reduce the production cost as a result.
As explained above, the apparatus for growing a single crystal of the present invention comprises the cooling cylinder that extends at least from the ceiling of the main chamber toward a raw material melt surface so as to surround a single crystal under pulling and is forcibly cooed with a cooling medium, and the auxiliary cooling member extending below the cooling cylinder and having a cylindrical shape or a shape tapered toward the downward direction. By using the combination of such a cooling cylinder and auxiliary cooling member, cooling effect can be exerted to the maximum extent, and hence the pulling rate can be increased to dramatically increase the productivity of single crystal. Further, since the auxiliary cooling member not directly cooled by the cooling medium or the like is provided below the cooling cylinder, the cooling cylinder is protected without the risk of contact with the melt at an extremely high temperature, and thus safety is further secured.